Aerial booms which are used to lift workers to elevated heights for working on electrical transmission lines generally include special electrically insulating boom sections. These insulating sections operate to insure that there is no electrical path from the top of the boom to ground. The insulating portion of the boom allows a worker to work directly on the line as the worker is at the same electrical potential. If the electrical resistance of the insulating boom portion breaks down, a worker could experience electric shock and injury.
The device shown in U.S. Pat. No. 3,320,524 attempts to minimize the effects of ambient electrical current by using a shielded cable between the conductive rings and the ammeter which measures the current flow. This however, does not eliminate the problem because the rings themselves are subject to the capacitive coupling. Thus, the readings at the ammeter do not reflect the actual resistive current leakage through the insulating portion of the boom.
Another system for testing the electrical integrity of an insulating boom portion is shown in U.S. Pat. No. 3,514,695. In this system, the bucket at the top end of the boom where the worker normally stands is placed in contact with an electrical line before the worker ascends. A pole extending from the bucket is placed in contact with the electrical line and current flow through the pole is measured. The current through the pole is considered to be equal to the flow through the insulating portion of the boom. The deficiency in this approach however, is the same as the system previously discussed. The reading does not give an acurate indication of current flow through the insulating portion on the boom when there are nearby sources of electrical energy, due to capacitive coupling.
U.S. Pat. No. 4,080,561 discloses a system for testing the electrical resistance of an insulating boom section. The system tests the boom in an out-of-service condition. This approach is not as good as monitoring electrical leakage through the boom before and during each use, as the resistance of the boom can change with varying conditions, such as condensation of water or cracking.
A system which attempts to minimize the effects of ambient electrical currents in measuring electrical leakage through an insulating boom portion is disclosed in U.S. Pat. No. 3,769,578. This system uses a pair of superimposed collector bands located on the interior and exterior of an insulation boom. These band are similar to those shown in U.S. Pat. Nos. 3,159,240 and 3,320,524 previously discussed. However, this later patent also employs a guard electrode which extends around the outside of the outer band. The guard electrode is grounded but is not in electrical contact with the other bands. The purpose of the guard electrode is to absorb ambient currents so that the current detected by the bands is only the electrical current which passes through the insulating boom portion. A deficiency of this system is that the guard electrode cannot fully shield the underlying collector bands from ambient currents and readings of leakage current through the boom may be inaccurate. A more serious problem is that applicants observation of a similar system has disclosed that a portion of the leakage current traveling through the insulating boom portion can be drained off by the guard electrode so that current actually passing through the boom may go undetected.
In addition to aerial booms, there are many other applications in electrical systems in which electrically resistive structures are used. It is usually desirable to keep leakage current through such structures to a minimum and structures with high leakage should be repaired or replaced. Because these structures are often located in areas of high ambient capacitive coupling, there has been no effective and accurate ways to measure the amount of resistive current leakage.
Thus, there exists a need in the prior art for an apparatus and method that can be used to separate the resistive leakage current through an electrically resistive structure for measurement, that is not effected by the presence of ambient capacitive coupling.